Bimetallic Catalysts For Enantioselective Epoxide Polymerization: Establishing And Using Mechanistic Hypotheses To Develop Enhanced Catalyst Systems

Abstract

In 2008, the Coates group reported a bimetallic cobalt catalyst that enantioselectively polymerized terminal epoxides to form highly isotactic polyethers and enantiopure epoxides. The complex catalyst system, which consisted of the catalyst and a cocatalyst, was extremely difficult to study due to the paramagnetism of the catalyst, short reaction times, exothermic nature of the reaction, induction periods, and precipitation of polyether during polymerization. Despite these challenges, a viable mechanistic hypothesis for the enchainment of epoxides was established using experimental observations and theoretical calculations focused on the structural features of the catalyst, the oxidation state of the metal center, the role of the cocatalyst, and free-energy changes during propagation of epoxide. The mechanistic insight gained was used to develop enhanced catalysts through systematic ligand variations, enabling higher activity and selectivity for isotactic polyether synthesis under milder and more controlled reaction conditions.